The art of manufacturing organosilane polysulfides is well established, with the art offering a variety of processing strategies.
Meyer-Simon, Schwarze, Thurn, and Michel disclose the reaction of a metal polysulfide with an .OMEGA.-chloroalkyltrialkoxysilane in U.S. Pat. No. 3,842,111. Example 2 shows the preparation of 3,3'-bis (triethoxysilylpropyl) tetrasulfide by reacting Na.sub.2 S.sub.4 with 3-chloropropyltriethoxysilane in absolute ethanol. The procedure for preparing the metal polysulfide is not exemplified, and the examples imply that this starting material is an isolated compound.
In U.S. Pat. No. 4,072,701, Pletka describes the preparation of sulfur-containing organosilicon compounds by first heating 3-chloropropyltrichlorosilane (Example 1) with ethanol, and then adding both sulfur and NaSH in the presence of alcohol. The reaction developed gaseous hydrogen sulfide in situ, but some of the sulfur therein was not recoverable (see, U.S. Pat. No. 4,129,585, col. 1, lines 32-34, in this regard). Therefore, the yields based on added sulfur tended to be low. Also, the use of NaSH is problematic due to its deliquescent nature and its tendency to oxidize to sulfate. The deliquescence is troublesome from the standpoint that it increases the risk that water will enter the reaction and cause hydrolysis of the alkoxide reactants.
After describing the above two patents in U.S. Pat. No. 4,129,585, Buder, Pletka, Michel, Schwarz and Dusing, describe a procedure for making the noted compounds without the production of gaseous hydrogen sulfide. The process entails reacting a suitable alkali metal alcoholate, e.g., sodium ethoxide, in preferably alcoholic solution with a desired .OMEGA.-chloroalkyltrialkoxysilane, a suitable metal hydrogen sulfide, and sulfur. The resulting product was purified by separating the salt formed and distilling off the alcohol. Again, the use of the metal hydrogen sulfide can be a source of water entering the system unless precautions are taken.
In U.S. Pat. No. 4,507,490, Panster, Michel, Kleinschmidt and Deschler, first prepare Na.sub.2 S. Again, they employ a metal hydrogen sulfide but react it with an alkali metal, such as sodium, in a polar solvent, such as ethanol. This reaction is highly exothermic and evolves hydrogen gas. The process is said to eliminate the use of an alkali metal alcoholate solution, noting that its production requires such a great deal of time as to be industrially improbable. The Na.sub.2 S is reacted with additional sulfur to form a desired polysulfide, preferably Na.sub.2 S.sub.4. The polysulfide is then reacted with a desired .OMEGA.-chloroalkyl trialkoxysilane, e.g., Cl(CH.sub.2).sub.3 Si(OC.sub.2 H.sub.5).sub.3, to form the desired .OMEGA.,.OMEGA.'-bis (trialkoxysilylalkyl) polysulfide.
Janssen and Steffen, in U.S. Pat. No. 3,946,059, offer a distinct approach and criticize procedures of the type described above. They eliminate the production, and therefore separation, of salts formed in the above reactions by contacting a bis (alkylalkoxysilyl) disulfide with sulfur at a temperature between 100.degree. and 200.degree. C. This procedure, however, adds the difficulty of the high temperature processing and requires the initial preparation of bis-silyl disulfides by the reaction of sulfuryl chloride with silyl mercaptans.
While the possibility might appear to exist that commercial forms of alkali metal sulfides, e.g., sodium tetrasulfide, could be employed, this would not be practical. The commercial forms of sodium tetrasulfide include water which must be completely removed prior to contact with the alkoxylates. If water is present, the alkoxide is hydrolyzed and a polysiloxane polymer is formed. And, while Bittner, et al. teach in U.S. Pat. No. 4,640,832, the reaction of sodium salts with hydrogen sulfide in alcoholic solution, this route has been criticized as "quite inconvenient" (see Lichty at CA, 7, 2910 (1913)).
Thus, the prior art has found the use of hydrogen sulfide gas, the separation of sodium chloride and the preparation of metal alkoxylates to be problematic in the preparation of sulfur-containing organosilicon compounds, and did not recognize that there was possible a reaction scheme which efficiently and effectively combines all of them. The invention provides a process which combines these and still obtains high yields based on sulfur.